1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication networks typically implement a main database to store subscriber information such as the International Mobile Subscriber Identities (IMSIs) for the users that are registered for communication within the system. The database can also include other information about the user, the users account, and/or user preferences for various communications. One example of a main database is a home location register (HLR) that can be maintained by the subscriber's home carrier or another network operator. The home location register typically interacts with a mobile switching center to control and/or process calls associated with the subscribers. Similar databases can also be used to temporarily store information related to mobile units or user equipment that have roamed out of their home network and into a visited network. One example of a visitor database is a visiting location register (VLR) that maintains temporary information for subscribers that are roaming into the visited network.
Evolved packet-switched networks such as Long Term Evolution (LTE) networks are being developed in accordance with 3GPP standards and/or protocols. The System Architecture Evolution (SAE) is an example of a core network architecture for an all-IP network. The SAE implements an Evolved Packet Core (EPC) that handles mobility within the network. One subcomponent of the EPC is a mobility management entity (MME) that is responsible for procedures in the idle mode such as tracking and paging mobile units including retransmissions. The MME can interact with a home subscriber server to authenticate mobile units and can terminate non-access stratum (NAS) signaling. The MME can also generate and allocate temporary identities to roaming mobile units. The MME can function as the termination point in the network for ciphering/integrity protection for NAS signaling. The MME and the VLR communicate over an interface that is conventionally referred to as an SGs interface.
When a mobile unit roams into an area served by a particular MME, the MME and the appropriate VLR establish an association that allows a mobile unit to send and receive messages via the MME and the VLR. Both the MME and the VLR include data that defines and identifies the association. Once an SGs Association has been established between the MME and the VLR, the MME and/or the VLR can use the SGs Interface to transmit uplink and/or downlink NAS messages associated with the mobile unit. For example, a mobile-originated short message service (SMS-MO) message can be transmitted from the mobile unit to the MME as a payload of an NAS message using an SGs-UPLINK-UNITDATA message. The MME uses the association data to transmit this message to the VLR, which uses its copy of the association data and the related user information to forward the SMS-MO message. For another example, a mobile-terminated SMS message can be received at the VLR, which uses its copy of the association data and the related user information to convey the SMS-MT message to the appropriate MME for forwarding to the mobile unit.
Various errors and/or network operations may cause full or partial data loss on either the MME side or VLR side of the SGs interface. The full or partial data loss can cause a loss of synchronization between the association data and/or user information data stored by the MME and the serving VLR. For example, VLR restarts may result in complete loss of the association and/or user information data stored by the VLR. For another example, restarts of the MME may result in full data loss of the association and/or user information data stored by the MME. For yet another example, some or all of the IMSI records may be lost and/or corrupted in portions of a distributed VLR architecture. The lack of synchronization between the association and/or user information data store by the MME and the serving VLR may result in abnormal conditions during the exchange of messages, e.g., during the exchange of NAS messages such as SMS-MO and SMS-MT messages.